Comparison of three statistical models describing potato yield response to nitrogen fertilizer

Estimation of optimum fertilizer rates is of interest because of growing economic and environmental concerns. Optimum fertilizer rates can be determined by fitting statistical models to yield data collected from N fertilizer experiments. We evaluated quadratic, exponential, and square root models describing the yield response of potato (Solanum tuberosum L,) to six rates of N fertilization (0-250 kg N ha(-1)) with and without supplemental irrigation at four on-farm sites in each of three years (1995 to 1997) in New Brunswick, Canada. Economic optimum N rates (N-op) varied among sites and models. The proportion of variability (R-2) explained by the three models was similar. The quadratic model, however, calculated a greater N-op value (175 kg N ha(-1)) averaged over all sites than those calculated by the square root (123 kg N ha(-1)) and exponential (80 kg N ha(-1)) models. Regression residues of the quadratic model were closer to a normal distribution than those of the other two models, indicating a less systematic bias. Economic losses were greatest when the quadratic model was the most appropriate model, but the data were fitted to the exponential (loss of $204-240 ha(-1); all values in Canadian dollars) or square root model (loss of $58-201 ha(-1)), We conclude that the quadratic model is the most appropriate for describing the potato yield response to N fertilizer and predicting N-op for areas with a ratio of the cost of N fertilizer to the price of potatoes similar to that in Atlantic Canada.